Reforming of hydrocarbon fuels to make hydrogen is well known in the art. In a first stage, hydrocarbons are reacted with steam to make a mixture of hydrogen, carbon monoxide and other components, commonly referred to as the reformate, sometimes also referred to as syngas, particularly before a water-gas shift reaction is performed. In a second stage, known as the water-gas shift reaction, the reformate is treated with additional steam to convert most of the carbon monoxide to carbon dioxide and produce additional hydrogen. However, the shift reaction is an equilibrium reaction, and typically does not reduce the carbon monoxide content of the reformate to a level suitable for supplying to a PEM fuel cell. For a PEM fuel cell, it is necessary to further remove carbon monoxide from the hydrogen-rich reformate stream. It is known to further reduce the carbon monoxide content of hydrogen-rich reformate exiting a shift reactor by a so-called preferential oxidation (“PrOx”) reaction (also known as “selective oxidation”) effected in a suitable PrOx reactor. A PrOx reactor usually comprises a catalyst that promotes the selective oxidation of carbon monoxide to carbon dioxide by oxygen in the presence of the hydrogen, without oxidizing substantial quantities of the hydrogen itself. The preferential oxidation reaction is:CO+½O2→CO2  (1)Desirably, the amount of O2 used for the PrOx reaction will be no more than about two times the stoichiometric amount required to react the CO in the reformate. If the amount of O2 exceeds about two to three times the stoichiometric amount needed, excessive consumption of H2 results. On the other hand, if the amount of O2 is substantially less than about two times the stoichiometric amount needed, insufficient CO oxidation may occur, making the reformate unsuitable for use in a PEM fuel cell. The essence of the PrOx process is described in the literature, for example, in U.S. Pat. Nos. 1,366,176 and 1,375,932. Modern practice is described, for example, in, “Preferential Oxidation of CO over Pt/γ-Al2O3 and Au/α-Fe2O3: Reactor Design Calculations and Experimental Results” by M. J. Kahlich, et al. published in the Journal of New Materials for Electrochemical Systems, 1988 (pp. 39-46), and in U.S. Pat. No. 5,316,747 to Pow et al.
A wide variety of catalysts for promoting the PrOx reaction are known. Some are disclosed in the above references. In modern practice, such catalysts are often provided by commercial catalyst vendors, and their compositions are typically proprietary. The practitioner is instead provided with approximate temperature ranges for use, and some physical parameters. The properties of candidate catalysts have to be evaluated in the actual proposed design before final selection of a catalyst for development or production. Moreover, catalysts come in a wide variety of physical forms. In addition to the “classical” pellets and powders, which are typically porous to some extent, catalysts are also supplied on any of a large variety of supports. These may also be pellets, but also include monoliths, such as the ceramic and metal honeycombs used in automotive catalytic converters, metal and ceramic foams, and other monolithic forms.
PrOx reactions may be either (1) adiabatic (i.e., where the temperature of the reformate and the catalyst are allowed to rise during oxidation of the CO), or (2) isothermal (i.e., where the temperature of the reformate and the catalyst are maintained substantially constant during oxidation of the CO). The adiabatic PrOx process is typically effected via a number of sequential stages which progressively reduce the CO content. Temperature control is important at all stages, because if the temperature rises too much, methanation, hydrogen oxidation, or a reverse shift reaction can occur. The reverse shift reaction produces more undesirable CO, while methanation and hydrogen oxidation decrease system efficiencies.
The selectivity of the catalyst of the preferential oxidation reaction is dependent on temperature, typically decreasing in selectivity as the temperature rises. The activity of the catalyst is also temperature dependent, increasing with higher temperatures. Furthermore, the reaction is very slow below a threshold temperature. For this reason the temperature profile in a PrOx reactor is important in maximizing the oxidation of carbon monoxide while minimizing the undesired oxidation of the hydrogen gas in the mixed gas stream.
More particularly, when the PrOx catalyst temperature is less than a certain value, high levels of CO may bind to the catalytic site but fail to react, thereby inhibiting the catalyst's performance. When PrOx temperature increases beyond a certain point, catalyst selectivity decreases, and a higher equilibrium CO concentration results. Because of these multiple sensitivities of the reaction to temperature, there is for any catalyst a preferred temperature range for efficient operation. Moreover, to minimize catalyst volume, it is often desirable to perform a first step of the preferential oxidation at a higher temperature, for speed of reaction, and a final cleanup at a lower temperature, for selectivity and for minimum reverse shift.
The need for temperature control adds numerous complexities to the system. For example, multiple air lines, air distributors, air flow controllers, and reactor vessels, as disclosed, for example, in U.S. Pat. No. 5,874,051, add size and manufacturing cost to the reactor, and further highlight the need for a compact, efficient reactor design. Compactness, simplicity, and efficiency are particularly important in small scale PrOx reactors suitable for use in mobile and domestic-scale systems.
The present invention addresses the above problems and challenges, and provides other advantages, as will be understood by those in the art, in view of the following specification and claims.